1. Field of the Invention
The present invention relates to a passive matrix electroluminescence (EL) display apparatus in which organic EL devices are driven using pulse width modulation signals.
2. Description of the Related Art
Organic EL devices are ideal for thin configurations as they emit light and do not require the backlight that is required in liquid crystal displays, and they do not have restrictions in viewing angle. Thus, the application of organic EL devices is highly expected in the next generation of display devices.
As shown in an example in FIG. 1, an organic EL device 1 is formed from a hole-transport layer 5, which is formed from MTDATA (4,4'-bis(3-methylphenylphenylamino)biphenyl), an emissive layer 6, which is formed from TPD (4,4',4"-tris(3-methylphenylphenylamino)triphenylanine) and Rubrene, and an electron-transport layer 7, which is formed from Alq3, between an anode (first electrode) 3, which is formed from a transparent electrode, such as ITO, on a transparent glass substrate 2, and a cathode (second electrode) 4, which is formed from an MgIn alloy. Holes injected from the anode 3 and electrons injected from the cathode 4 are recombined within the emissive layer 6 to emit light, which is radiated outward from the transparent anode side in the direction of the arrow shown in the figure.
Display apparatuses for driving this sort of organic EL device can be divided into two types: a passive matrix type, and an active matrix type using TFTs. A schematic circuit diagram of the passive matrix type is shown in FIG. 2.
Namely, of the pair of electrodes of the EL devices described above, the anodes 3 are designated for columns, the cathode 4 are designated for rows, and they are arranged in a matrix configuration so as to cross each other and sandwich an organic layer. To the cathodes 4 are supplied scan signals ROW1, ROW2, ROW3, and so forth, from a row driver 8, the scan signal of only the selected row of a plurality of rows becomes a low level for one horizontal period while the scan signals for the other rows become a high level. Meanwhile, a column driver 9 inputs gray-scale data mDATA for expressing the display gray scale of each pixel, and pulse signals having pulse widths proportional to this gray-scale data are output as column driving signals COL1, COL2, COL3, . . . , COLm. The column driving signals COL1, COL2, COL3, . . . , COLm are at a high level during the pulse width period, thus, the EL device of the row that inputs the low level scan signal emits light.
The configuration of the column driver 9 will be described in detail with reference to FIG. 3.
The column driver 9 comprises a shift register 10 for inputting n-bit gray-scale data mDATA for each column according to a shift clock CL, a latch circuit 11 for latching the data input by the shift register 10 according to a latch pulse, an n-bit counter 12 for expressing the gray-scale level, and m pulse width modulation circuits 13 for comparing the n-bit gray-scale data from the latch circuit 11 provided for every column and the n-bit counter value, and respectively outputting the column driving signals COL1, COL2, COL3, . . . , COLm of pulse widths proportional to the gray-scale data. In the passive matrix EL display apparatus, the column driving signals COL1, COL2, COL3, . . . , COLm are output from the respective pulse width modulation circuits 13 as shown in FIG. 4.
The counter value of the n-bit counter 12, as shown in FIG. 4 when n=3, for example, changes in a sequence of "0", "1", . . . , "7" during one horizontal scan period (1H), and the column driving signals COL1, COL2, COL3, . . . , COLm all simultaneously start their output at a timing when the counter value reaches "1". The high level during the pulse width period is maintained in proportion to the gray-scale data of the respective pixel. Therefore, pixels PX1, PX2, PX3, . . . , PXm of the same row shown in FIG. 2 emit light during the pulse width periods shown in FIG. 4, and the gray scales are expressed by these light emitting periods.
In the above-mentioned EL display apparatus, gray scales are expressed by the pulse widths of the pulse width modulation signals that are output as the column driving signals COL1, COL2, COL3, . . . , COLm as described above, and the output start timing is the same for all signals. Therefore, at the initial timing when the counter value becomes "1", the current concentrates in its flow from the anode 3 to the cathode 4 to result in an extremely high current consumption at this time. However, the gray scale of the pixel is dependent on the high level period during one horizontal scan period and is not dependent on the generated position of the pulse width modulation signal.